In digital signal processing (DSP) of a high-rate communications system for polarization multiplexing coherent optical transmission, carrier phase noise compensation is a key technology. Increasingly high spectral efficiency requires that a signal be compressed in a narrower spectrum for transmission, thereby causing serious signal distortion and further increasing difficulty of carrier phase noise compensation. In addition, regarding metropolitan transmission, a 10 gigabits (Gb) on/off keying (OOK) modulation channel will exist on a network for a long term, causing serious non-linear carrier phase noise to a channel that uses coherent transmission. Therefore, resolving these problems in carrier phase noise compensation is of great significance to future polarization multiplexing coherent optical transmission, and in particular, to metropolitan polarization multiplexing coherent optical transmission.
In other approaches, a pilot-based carrier phase noise compensation method is generally used in a coherent communications system. Further, in this method, a transmit end inserts a pilot of a specific frequency into a signal, in DSP at a receive end, a digital band-pass filter first extracts the pilot from a signal carrying the pilot, phase noise is then estimated according to the pilot, and the phase noise is finally used to compensate the signal, to obtain a compensated signal.
However, there are two orthogonal state of polarization (SOP) signals in a polarization multiplexing system. If the pilot-based carrier phase noise compensation method used in the coherent communications system in the other approaches is applied to the polarization multiplexing system, pilots in the two SOP signals counteract each other at some specific polarization angles and differential group delay (DGD) values, because of impact of link SOP rotation and a DGD in the polarization multiplexing system. Consequently, a pilot in one SOP signal disappears, and a pilot in the other SOP signal is strengthened. As a result, a pilot extracted in a subsequent pilot extraction process is noise in the SOP signal whose pilot disappears. Therefore, phase estimation cannot be performed accurately, and performance of the polarization multiplexing system is degraded.